The present technique relates to medical imaging devices and, more particularly, to imaging devices with cryogenic cooling systems.
A number of important applications exist for superconductive magnet systems. These include imaging systems, as for medical imaging, as well as spectrometry systems, typically used in materials analysis and scientific research applications. The present technique relates to management of cryogenically cooled superconductive magnets, and particularly to the monitoring and servicing of such systems. Although reference is made throughout the following discussion to imaging systems, it should be borne in mind that the technique is applicable to a range of systems that utilize cryogenically cooled superconducting magnets.
Imaging devices are omnipresent in typical medical environments. Medical practitioners, such as physicians, may employ medical imaging devices to diagnose patients. Imaging devices, such as Magnet Resonance Imaging (MRI) devices and Nuclear Magnetic Resonance (NMR) devices, produce detailed images of a patient""s internal tissues and organs, thereby mitigating the need for invasive exploratory procedures and providing valuable tools for identifying and diagnosing disease and for verifying wellness.
Typical MRI and NMR devices develop diagnostic images by affecting gyromagnetic materials within a patient via controlled gradient magnetic fields and radiofrequency pulses in the presence of a main magnetic field developed by a superconductive magnet. During an MRI exam, a main magnetic field of upwards of two Tesla may be necessary to produce vivid images. Typically, superconductive electromagnets comprise loops of coiled wire, which are continuously bathed in a cryogen, such as liquid helium, at temperatures near absolute zero. For the example of bathing the coils with a liquid pool of helium, system temperatures are approximately xe2x88x92269xc2x0 C. (or 4 K) near atmospheric pressure (e.g. less than 5 psig). When cooled to such extreme temperatures, the coiled wire becomes superconductive, i.e., the electrical resistance of the wire falls to essentially zero, enhancing the field strength without requiring significant energy input for continued operation. Advantageously, superconductive electromagnets reduce the electrical load requirements for producing the desired magnetic fields, thereby making the MRI system more economical to operate.
Challenges exist, however, in maintaining the electromagnets at these extreme temperatures which are significantly lower than ambient temperatures. Because of this temperature difference with ambient, a considerable driving force exists for heat transfer from the environment into the magnet system. Accordingly, thermal insulating material and other heat transfer barriers, such as vacuum regions, may insulate the magnet and cryogen to impede heat transfer from the environment. For environmental heat effects that reach the inner workings of the magnet system, the liquid pool of cryogen that surrounds the magnet must absorb the heat to maintain the magnet at desired temperature. Cryogens operating at or near their boiling points typically expend this external heat by vaporizing relatively small amounts of cryogen.
In general, the cryogen liquid pool and its heat of vaporization consume heat while maintaining the magnet at constant temperature. On the whole, cryogen liquid pools in well-insulated systems, such as typical superconducting magnet systems, are able to absorb heat transferred from the environment over relatively long periods of time to maintain the magnet at desired temperature. Other systems with refrigerants operating below their boiling points (i.e., super-cooled) and which primarily absorb heat via sensible heat increases, typically require more refrigerant and processing of the refrigerant. Additionally, for liquid pools relative to other techniques, the magnet temperature, in some circumstances, may be better maintained constant because, in part, the cryogen boils at fairly constant temperatures at moderately fixed pressures.
Furthermore, liquid pools of cryogens may be suitable for superconducting applications because magnet temperature may be controlled by controlling cryogen pressure at a specified pressure that gives a liquid pool boiling temperature that corresponds to the desired magnet temperature. This may prove advantageous over direct control of temperature because pressure measurement may generally be a more economical and reliable application than temperature measurement. Additionally, because cryogens, such as helium, boil in the desired lower temperature ranges at slightly positive pressures (e.g., near atmospheric at 0-5 psig), vacuum operating conditions are generally not needed to induce low boiling temperatures, thus permitting simpler and more economical system design and operation.
Cryogenic liquids, such as liquid helium, however, are relatively expensive to refine and maintain. Therefore, the aforementioned advantages of cryogen in superconducting magnets applications may be offset if cryogen losses are excessive. Accordingly, older approaches of xe2x80x9copenxe2x80x9d systems which have no recondensing capability and where cryogen vapor is normally vented to the atmosphere, have generally fallen out of favor in the industry. In these systems, as the liquid cryogen absorbs environmental heat in maintaining the desired magnet temperature, vaporized cryogen is normally vented to limit pressure increases and thus to limit temperature increases. The simplicity, however, of relying solely on a vent or relief device to control the high end of cryogen pressure is usually offset by additional costs and downtime of servicing and refilling the cryogen system.
While the economic operation of the system is desirable, there is the environmental consideration driven by the reality that helium is a finite natural resource.
Once extracted from the ground (helium is a refined by-product of natural gas extraction) it is not replenished. The utmost care must be employed to ensure closed systems remain closed. Once vented by the relief valve in an overpressure condition OR accidentally by an unintentional plumbing leak those molecules are gone forever into the atmosphere. Helium""s density being lighter than most other elements rises, and because of this does not remain at ground level in sufficient concentration for atmospheric extraction and re-processing.
Therefore, to conserve cryogen, such as helium, and to support cryogen pressure control, magnet systems in typical MRI devices may now include a cryogen condensing system, which recondenses volatilized cryogen back into its liquid phase. That is, cryogen is maintained in a sealed cryogen vessel (or cryostat) that provides cryogen vapor (i.e., gaseous helium) to the condensing system and receives liquid cryogen (i.e., liquid helium) from the condensing system in a closed loop process. The condensing system condenses cryogen vapor, thus recovering the vapor, as well as, maintaining the cryogen pressure below the set point of the vent or relief device. On the flip side, as discussed more below, a heater may be used to prevent the cryogen pressure from dropping too low. In sum, for the older open systems, a loss in cryogen level is expected and the timing of service intervals is typically based on this loss of level. In contrast, for recondensing magnets systems which recover the vaporized cryogen, losses in cryogen liquid level are not expected during normal operation. Thus, recondensing magnet systems generally retain cryogen level and reduce the requirement of periodic refilling of cryogen.
Recondensing magnet systems, however, from time to time, require maintenance, for example, when the cryogen condensing system may require repair or replacement. In particular, the performance of the condensing system components will degrade due to wear, thereby reducing the efficacy of the condensing system and overall magnet cooling system (cryogenic cooling system). Moreover, leaks within the cryogen (helium) vessel and/or condensing system, again for example, may also reduce the efficacy of the cooling system. During maintenance, it may become necessary to disengage the condensing system, cooling system, and/or deactivate the MRI devices, events that are to be avoided. If the cryogen condensing system is off-line or not condensing effectively, more of the liquid cryogen may begin to volatilize, leading to an increase of pressure in the cryogen vessel (i.e., cryostat). To prevent adverse effects due to the increased pressure, traditional devices, such as a relief valve is installed, for example, on the cryogen vessel to relieve pressure by venting some of the gaseous cryogen to the atmosphere. This conversion of liquid cryogen, such as liquid helium, to its gaseous state, and/or the subsequent venting of the gas, is generally known in the industry as xe2x80x9cboil-off.xe2x80x9d Venting of the gas leads to permanent loss of expensive cryogen, requiring refilling of the system. Again, this venting is expected during normal operation of open systems but may be substantially avoided in recondensing systems.
Recondensing technology may provide net xe2x80x9czero boil-offxe2x80x9d systems in which a refrigeration system or xe2x80x9ccold headxe2x80x9d (also called cryo-cooler or cryo-condenser) typically runs continuously to condense (and re-condense) vaporized cryogen. An electric heater in the vessel heats the cryogen to maintain a desired pressure level, thereby preventing the vessel pressure from falling below a desired level that could result in drawing atmospheric gases into the vessel. A balance is maintained between cooling components (condensing) and heating components (vaporizing) which can be continuously monitored. In contrast, with xe2x80x9copen loopxe2x80x9d thermal designs, such as those without recondensing technology, the expected boil-off results in a measurable drop in liquid level in the vessel over time. In some configurations, this drop in liquid level may manifest in a reduction in the reading, for example, on a provided liquid level gauge. Historically, with open systems, the percent rate of drop in a volumetric table unique to each magnet system (i.e., in each MRI system) defines the boil-off rate, for example, expressed as liters per hour consumption. For open systems, this boil-off rate is compared to design norms to determine total thermal system performance and used to determine service intervention. On the contrary, properly operating recondensing systems experience no loss of cryogen level, employing the refrigerating action of a cold head to provide controlled zero boil-off (C0BO) with very high percentage cryogen (helium) recovery. Though true zero boil-off systems are not physically possible, the operation of condensing the helium vapor and returning the condensed helium to the helium vessel (cryostat) liquid pool may approach ideal conditions.
A problem is that the previous methods of evaluating thermal system performance which depended upon observing a drop in liquid level and then comparing the results to design norms are generally not adequate for recondensing systems. During normal and early failure modes, recondensing thermal systems, by design, exhibit no drop in liquid level over time thereby preventing prompt identification failure until the problem becomes severe enough, for example, that the pressure relief valve opens the previously closed system and gas is vented resulting in the liquid level dropping. It should be emphasized that level based methods of evaluating magnet thermal performance may be inadequate to fully understand and respond to problems affecting recondensing magnet performance.
Traditionally, the maintenance of cooling systems in MRI devices is a reactive process. That is, technicians are generally called when, for example, image quality has been affected, a critical indicator has activated, and/or the system is no longer operable. For example, a typical system may generate a service call when a low level of cryogen is detected due to venting or leaks in the system. In addressing concerns reactively, the repair time and/or off-line periods may be longer than desired. For example, certain parts and/or technicians may not be immediately available, leading to longer than necessary downtimes (i.e., off-line time). Moreover, periods of reactive maintenance may not coincide with already scheduled routine maintenance procedures, leading to duplicative downtimes for the MRI device. Similarly, when substantial quantities of cryogen are required, very significant costs may be incurred in refilling the serviced system.
Similar problems exist even prior to the time such magnets are placed in operation. For example, magnets are typically built and tested in a controlled factory environment, then at least partially disassembled from other support equipment for shipping. Current procedures for building, testing and shipping superconductive magnets do not, however, adequately accommodate boil-off or servicing needs. In much the same way, mobile MRI systems and systems where communications infrastructures are less available pose particular challenges beyond those of traditional fixed locations in hospitals. Such challenges include cryogen monitoring and servicing, but also location and identification of the systems, and communication of relevant parameter data to a monitoring or service-coordinating location.
Accordingly, there is a need for an improved technique for maintaining cryogen cooling systems. Particularly, there is a need for a technique that reduces maintenance times, periods of deactivation, costs, and so forth.
The present invention provides a novel technique designed to respond to such needs. The technique provides for observation of duty cycle of the active pressure control circuit (i.e., the heater duty cycle) to determine total thermal system performance. The technique may accommodate a multitude of variables that may contribute to non-zero boil-off operation of recondensing thermal systems. Such variables may include, for example, degrading cooling capacity of the cold head (cryo-cooler machine) over its operation life, plumbing leaks in the pressure system that violate the close system requirement, environmental effects such as facility issues preventing optimal cold head (cryo-cooler system) functionality, and so forth. The technique provides earlier possible identification of failures related to such variables, as well as, facilitates identification of the nature or root cause of the problem. The technique may provide for relying on observation of the effects, such as abnormal heater duty cycle, or of problems, such as reduced cooling capacity, early enough in the failure cycle to facilitate predictive maintenance, reduce maintenance costs, reduce downtime, prevent helium loss, and the like. Monitoring of the heater duty cycle (e.g., energization time) may offer advantages, such as improved predicted maintenance, over the traditional approach of monitoring or alarming on low level in the cryogen (helium) vessel. It also may offer similar advantages over monitoring pressure in the cryogen vessel.
Aspects of the invention, for example, provide a method for operating a superconducting magnet system, including monitoring a duty cycle of a heating element that supplies heat to the superconducting magnet system, comparing the duty cycle to a predetermined value, and providing an indication of a condition of the magnet system based upon the comparison. The duty cycle may represent the periods of time that the heating element is energized and may be expressed in percent of the time the heating element is energized. The periods of time may be non-uniform lengths of time with a substantially constant amplitude of energization and with the heating element configured in an on/off control scheme (a constant heater when energized). The predetermined value of the duty cycle generally corresponds to an operating norm of the duty cycle and an exemplary specified tolerance is 50 percent of the predetermined value. The method may further include notifying a technician or alarming when the duty cycle falls outside a tolerance. The duty cycle may be monitored remotely and used as a variable to determine when the superconducting magnet system is to be serviced. The method may further identify the root causes of changes in the duty cycle. The amplitude of energization may vary and the heating element may be a variable heater in a proportional-integral-derivative (PID) control scheme. In general, the amplitude of energization may vary and the periods of time the heating element is energized may be constant. Moreover, the superconducting magnet system may provide one or more magnetic fields in a magnetic resonance imaging (MRI) system. Finally, the pressure in the cryogen vessel in the superconducting magnet system may be monitored and used as a variable in determining when the superconducting magnet system is to be serviced.
Other aspects of the invention provide a method for monitoring a superconducting magnet system, including monitoring an energization of a heating element that supplies heat to a superconducting magnet system, comparing the energization to a specified tolerance, and indicating when the energization falls outside the specified tolerance. The energization may be controlled, monitored remotely, and used as a variable to determine when the superconducting magnet system is to be serviced. Additionally, indicating when the energization falls outside the specified tolerance may include at least one indicia of a signal to an indicator or indicator system, an audible alarm, a signal to a control system, an indicating light on a graphical-user interface, and an electronic message. Further, the superconducting magnet system is repaired or re-configured in response to indication of the energization falling outside of the specified tolerance and to conform the energization to within the specified tolerance. Moreover, the superconducting magnet system provides one or more magnetic fields in at least one of a magnetic resonance imaging (MRI) system, nuclear magnetic resonance (NMR) system, and a spectroscopy system.
Yet other aspects of the invention provide a method for operating a superconducting magnet system, including monitoring a duty cycle of a pressure control circuit that controls pressure in a cryogen vessel in the superconducting magnet system, comparing the duty cycle to a predetermined value, and providing an indication of a condition of the magnet system based upon the comparison. The cryogen vessel holds a helium liquid pool that surrounds one or more magnets in the superconducting magnet system.
Facets of the invention provides a superconducting magnet system, including a cryogen vessel that contains or surrounds one or more magnets disposed in the superconducting magnet system, a heating element disposed within the cryogen vessel, a heater controller for controlling energization of the heating element which vaporizes a cryogen liquid disposed in the cryogen vessel, a monitoring system comprising one or more interfaces and one or more sensors for remotely monitoring the energization of the heating element, and one or more indicators for indicating when the energization falls outside a predetermined tolerance. A relief device may be disposed on or near the cryogen vessel. Moreover, the energization of the heating element may be adjusted to control pressure in the cryogen vessel and to control temperature of the one or more magnets. The cryogen liquid may comprise helium liquid, the pressure of the cryogen vessel may be controlled in the approximate range of 4.0 to 4.5 psig, and the temperature of the magnets may be controlled at approximately 4 Kelvin. The system may include a cold head for condensing cryogen vapor from the cryogen vessel, and wherein operation of the cold head affects cryogen pressure and thus affects the temperature of the one or more magnets. Moreover, the heater controller may be configured for at a constant heater in an on/off control scheme with constant amplitude energization while the heater is on and the heating element is energized, and wherein the periods of time the heating element is energized are non-uniform in length of time. On the other hand, the heater controller may be configured for a variable heater in a proportional-integral-derivative (PID) control scheme with variable amplitude energization of the heating element.
In accordance with aspects of the invention, a superconducting magnet system includes a heater controller that controls an energization rate of a heating element disposed within the superconducting magnet system, a control system with one or more interfaces and one or more sensors for remotely monitoring the energization rate of the heating element; and one or more indicators for indicating when the energization rate of the heating element falls outside a specified tolerance. The superconducting magnet system may be disposed within a magnetic resonance (MR) imaging system, and the one or more interfaces for monitoring the energization of the heating element may include at least one of a laptop, a computer, a workstation, a network connection, and a MR imaging system interface. The system may further include a cryogen vessel disposed in the superconducting magnet system and holding a cryogen at its boiling point for cooling one or more magnets, a cold head configured for removing heat from the superconducting magnet and condensing cryogen vapor received from a vapor space of the cryogen vessel, a refrigerant compressor system that supplies refrigerant to the cold head to cool the cold head, and a relief vent disposed in the superconducting magnet system for relieving cryogen vapor to control pressure and temperature within the superconducting magnet system.
In accordance with other aspects of the invention, a system for operating a superconducting magnet system includes means for controlling and adjusting the energization of a heating element that supplies heat to the superconducting magnet system, means for monitoring the energization of the heating element, means for comparing the energization of the heating element to a predetermined value, and means for providing an indication of a condition of the magnet system based upon the comparison. The means for controlling energization may control the temperature of one or more magnets disposed in the superconducting magnet system by controlling pressure within a cryogen vessel that surrounds the one or more magnets. Additionally, the system may have means for remotely monitoring the energization of the heating element.
Another example is a system having means for notifying a technician when the energization falls outside a specified tolerance, means for identifying one or more root causes of changes in the energization, means for conforming the energization of the heating element to within the specified tolerance, means for relieving pressure and cryogen vapor from the superconducting magnet system, and means for condensing the cryogen vapor disposed within the superconducting magnet system and for removing heat from the superconducting magnet system. One embodiment includes a system for monitoring a superconducting magnet system having means for monitoring a duty cycle of a heating element that supplies heat to the superconducting magnet system, means for comparing the duty cycle to a predetermined value, and means for providing an indication of a condition of the magnet system based upon the comparison. The superconducting magnet system may be disposed in a magnetic resonance imaging (MRI) system, nuclear magnetic resonance (NMR) system, a spectroscopy system, and the like. Moreover, a computer program, provided on one or more tangible media, for operating a superconducting magnet system, may include a routine for controlling and adjusting the energization of a heating element that supplies heat to the superconducting magnet system, a routine for remotely monitoring the energization of the heating element, a routine for comparing the energization of the heating element to a predetermined value, a routine for providing an indication of a condition of the magnet system based upon the comparison, and a routine for notifying a technician when the energization falls outside a specified tolerance. Finally, another computer program, also provided on one or more tangible media, for monitoring a superconducting magnet system, may include a routine for monitoring a duty cycle of a heating element that supplies heat to the superconducting magnet system, a routine for comparing the duty cycle to a predetermined value, a routine for providing an indication of a condition of the magnet system based upon the comparison, and a routine for controlling energization of the heating element.